1. Field of the invention
The present invention relates, in general, to a method for fabricating a piezoelectric/electrostrictive thick film using a seeding layer and, more particularly, to an improvement in the bonding properties between a substrate and a piezoelectric/electrostrictive film, along with the method.
2. Description of the Prior Art
In fabricating various film-type devices with ceramics, typically used is ceramic sol with a controlled viscosity or suitably modified ceramic powder. When the ceramic sol or ceramic powder is applied on substrates to give piezoelectric/electrostrictive films, especially, thick films, the solid bonding between substrates and piezoelectric/electrostrictive films is one of the most important factors which determine the quality of the devices fabricated.
Where the materials of the substrates are of the same as or similarity to the ceramics applied, no problems occur in the bonding between substrates and piezoelectric/electrostrictive films. However, where the materials of the substrates are non-ceramic or where the materials, even if they are ceramic, are different in crystal structure, surface property and reactivity from the ceramics applied, a poor bonding results therebetween. If so, the resulting device may suffer from a variety of problems, including separation of the piezoelectric/electrostrictive film from the substrate, progression of cracking in the piezoelectric/electrostrictive film in the vertical direction to the piezoelectric/electrostrictive film, and degradation acceleration of the ceramic material with the repetitive use of the device.
Particularly, as the piezoelectric/electrostrictive film is formed more thickly, the mechanical stress which is generated upon drying the piezoelectric/electrostrictive film has a greater influence on the bonding between the substrate and the piezoelectric/electrostrictive film. For piezoelectric/electrostrictive thick film, therefore, more significant account must be taken of improving the bonding property between a substrate and a piezoelectric/electrostrictive film.
Conventionally, in order to secure the solid bonding between a substrate and a piezoelectric/electrostrictive film and to prevent undesirable reactions at the interface therebetween, various attempts have been made: for example, a buffer layer is inserted between a substrate and a piezoelectric/electrostrictive film; a small thickness is set to the film to be formed in one round, reducing the mechanical stress; and an adhesive is used to bond a bulk to a substrate.
Of them, the recruitment of a buffer layer is representative. According to this technique, a metal thin film is deposited on a substrate by a sol-gel coating method, a sputtering method or an electrochemical method and thermally treated to give a buffer layer, followed by the formation of a piezoelectric/electrostrictive film on the buffer layer. As the buffer layer, a metal film such as titanium or platinum is typically used.
In order to better understand the background of the invention, a description will be given of such conventional methods in conjunction with the accompanying drawings.
With reference to FIG. 1, there is illustrated a fabrication method of a piezoelectric/electrostrictive film using a buffer layer. As shown in this figure, on a substrate is deposited a metal layer, such as titanium or platinum, which is subsequently subjected to thermal treatment to give a buffer layer. Then, a piezoelectric/electrostrictive film is formed on the buffer layer.
Illustrated in FIG. 2 is the same process as in FIG. 1 except that a substrate is thermally treated in advance of the formation of a buffer layer. In detail, the silicon substrate undergoes thermal oxidation to grow a silicon dioxide (SiO2) layer atop which a titanium or platinum thin film is then deposited. A thermal treatment allows the metal thin film to be used as a buffer. Subsequently, the buffer layer is overlaid by a piezoelectric/electrostrictive film.
The thermal treatments at high temperatures for the surface treatment of the substrate or the formation of the buffer make the above-mentioned techniques complicated. In addition, the techniques suffer from a disadvantage in that the resulting devices are poor in stability owing to the repetitive thermal treatments at high temperatures.
Referring to FIG. 3, there is a mechanical stress reducing technique of mechanical stress which is conducted in such a way that a film fraction to be formed in one round is as thin as possible. According to this technique illustrated in FIG. 3, a piezoelectric/electrostrictive thin film with a thickness of hundreds nm is deposited or coated over a substrate and thermally treated at high temperatures and this procedure is repeated until a desired thickness is obtained. When a piezoelectric/electrostrictive thick film is formed by this technique, the repetitive thermal history forces the initially formed piezoelectric/electrostrictive film fractions to be diffused over the substrate. This diffusion of piezoelectric/electrostrictive film gives an advantage of strengthening the bonding between the piezoelectric/electrostrictive film and the substrate, but suffers from a disadvantage in that an interfacial reaction therebetween occurs to denature the materials used or to degenerate their physical properties. In addition, a limit resides in the film thickness which can be obtained by such a repetitive film forming process.
Another technique is to use an adhesive to bond a piezoelectric/electrostrictive film on a substrate. Usually, this technique is used where a piezoelectric/electrostrictive film, consisting of ceramic bulk, is separately constructed and bonded to a substrate. As shown in FIG. 4, this technique is conducted by coating an adhesive, usually a thermocuring adhesive, on a substrate, applying a piezoelectric/electrostrictive film, separately formed and sintered, to the adhesive, and curing the adhesive to firmly fix the film and the substrate. The adhesive layer thus formed is excessively thick, amounting, in thickness, to 7-15 xcexcm. In addition, the adhesive layer tends to be non-uniform in its surface or may contain pores therein which bring depreciation in quality and production yield to the devices.
It is well known to use a seeding layer in forming a piezoelectric/electrostrictive film. However, the conventional methods are confined within a boundary in that the piezoelectric/electrostrictive thin film is formed at a thickness of 1 xcexcm or less with the aid of a sol-gel multiple coating process, a dry process such as sputtering, or a electro-crystallization process. The conventional methods cannot be used where a printing process or molding process is applied or a piezoelectric/electrostrictive thick film is formed.
Therefore, it is an object of the present invention to overcome the above problems encountered in prior arts and to provide a method for fabricating a piezoelectric/electrostrictive thick film, by which a solid bonding is formed between a piezoelectric/electrostrictive film and a substrate, improving the reliability of the resulting devices.
In accordance with an aspect of the present invention, there is provided a method for fabricating a piezoelectric/electrostrictive thick film, using a seeding layer, comprising the steps of: providing a substrate; forming on the substrate the seeding layer from a first ceramic sol solution or a ceramic paste, both identical or similar in composition to the piezoelectric/electrostrictive film, said ceramic paste being prepared from a mixture of a ceramic oxide powder, which has a particle size of 5 xcexcm or less and is prepared from Pb and Ti-based piezoelectric/electrostrictive elements by a non-explosive oxidation-reduction combustion reaction at 100-500xc2x0 C., and a second ceramic sol solution in water or an organic solvent, identical or similar in composition to said ceramic oxide powder; subjecting the seeding layer to an after-treatment to improve the reactivity of the seeding layer and to prevent undesirable reaction between the seeding layer and the substrate; forming a piezoelectric/electrostrictive film on the seeding layer to a desirable thickness; and thermally treating the piezoelectric/electrostrictive film and the seeding layer at 100-600xc2x0 C. to sinter the seeding layer and the piezoelectric/electrostrictive film and to solidify the bonding between the film and the substrate, whereby the piezoelectric/electrostrictive thick film is allowed to show a superb bonding to the substrate in the low temperature thermal treatment.
In accordance with another aspect of the present invention, there is provided a method for fabricating a piezoelectric/electrostrictive thick film, using a seeding layer, comprising the steps of: providing a substrate; forming on the substrate the seeding layer from a first ceramic sol solution or a ceramic paste, both identical or similar in composition to the piezoelectric/electrostrictive film, said ceramic paste being prepared from a mixture of a ceramic oxide powder, which has a particle size of 5 xcexcm or less and is prepared from Pb and Ti-based piezoelectric/electrostrictive elements by a non-explosive oxidation-reduction combustion reaction at 100-500xc2x0 C., and a second ceramic sol solution in water or an organic solvent, identical or similar in composition to said ceramic oxide powder; partially drying said seeding layer at room temperature to 150xc2x0 C. to provide said seeding layer with a proper viscosity for bonding; attaching on said partially dried seeding layer a piezoelectric/electrostrictive film, said piezoelectric/electrostrictive film being separately formed from a ceramic oxide powder which has a particle size of 5 xcexcm or less and is prepared from Pb and Ti-based piezoelectric/electrostrictive elements by a non-explosive oxidation-reduction combustion reaction at a low temperature of 100-500xc2x0 C.; and thermally treating said piezoelectric/electrostrictive film at 100 to 600xc2x0 C. to sinter the seeding layer and the piezoelectric/electrostrictive film and to solidify the bonding between the film and the substrate, whereby the piezoelectric/electrostrictive thick film is allowed to show a superb bonding to the substrate in the low temperature thermal treatment.
In accordance with a further aspect of the present invention, there is provided a method for fabricating a piezoelectric/electrostrictive thick film, using a seeding layer, comprising the steps of: providing a substrate; forming on the substrate the seeding layer from a first ceramic sol solution or a ceramic paste, both identical or similar in composition to the piezoelectric/electrostrictive film, said ceramic paste being prepared from a mixture of a ceramic oxide powder, which has a particle size of 5 xcexcm or less and is prepared from Pb and Ti-based piezoelectric/electrostrictive elements by a non-explosive oxidation-reduction combustion reaction at 100-500xc2x0 C., and a second ceramic sol solution in water or an organic solvent, identical or similar in composition to said ceramic oxide powder; partially drying said seeding layer at room temperature to 150xc2x0 C. to provide said seeding layer with a proper viscosity for bonding; attaching a piezoelectric/electrostrictive film, separately molded and sintered, on said partially dried seeding layer; and thermally treating said piezoelectric/electrostrictive film at 100 to 600xc2x0 C. to sinter the seeding layer and the piezoelectric/electrostrictive film and to solidify the bonding between the film and the substrate, whereby the piezoelectric/electrostrictive thick film is allowed to show a superb bonding to the substrate in the low temperature thermal treatment.